Many common surfactants used in cleaning compositions are difficult to handle in concentrated form. In particular, it is well-known that some surfactants such as alkyl sulfates and alkyl ether sulfates exhibit a prohibitively viscous gel phase or “middle phase” for aqueous concentrations in the range of roughly 30% to 60% by weight surfactant, while exhibiting a thick but flowable lamellar phase at somewhat higher concentrations.
To save on transportation and storage costs, it is preferable to handle these materials in a concentrated form. However, in order to dilute the lamellar phase into the isotropic phase, care must be taken to avoid generation of the middle phase or mesophase. Once the middle phase is formed, it can take hours to days to dilute this high-viscous phase further due to the slow mixing dynamics, rendering dilution of the lamellar phase via simple impeller mixing as impractical on an industrial scale. Often, a high-energy device is employed to break up local regions of intermediate compositions before they can form the difficult middle phase, and care must be taken in the order of ingredient addition to avoid compositions that lie in the middle phase.
Several approaches have been disclosed in the art for adding a second material to the lamellar surfactant to mitigate the middle phase, usually a hydrotrope such as that discussed in U.S. Pat. No. 5,635,466, but other surfactants such as that discussed in U.S. Pat. No. 5,958,868 and micronized air such as that discussed in JP 2002-038200A have also been disclosed to be effective in some narrow applications.
In most cases, where the addition of another material to mitigate the middle phase is not desirable, the common solution is to dilute the lamellar phase very carefully into water using a specialized dilutor, such as a Bran-Luebbe as disclosed in Seifen, Oele, Fette, Wachse (1977), 103(16), 465-6 CODEN: SOFWAF: ISSN 0173-5500. In this operation, specialized pumps deliver the water and lamellar surfactant at a precise flow ratio into a high-shear device to dilute the surfactant to a fixed concentration, typically ˜25%. This approach of high-shear dilution into water can be extended to lamellar surfactant blends as discussed in US 2008/0139434A1; however, using this blend unnecessarily fixes the compositional ratio of the blended surfactants across all the products to be made at a particular location. Very specialized pumps are required due to the low viscosity of the water phase, the high viscosity of the surfactant phase, and the need to strictly avoid flow ratios resulting in a composition in the middle phase of the phase diagram. In fact, in some situations, the need for a specialized dilution system outweighs the cost savings of transporting the surfactant in the high-active form to the cleaning-product manufacturing facility, and thus the surfactant is created only in the diluted form.
It is of interest to note that in all lamellar surfactant dilution processes disclosed in the art, the diluting medium is primarily water, presumably because other ingredients present in the aqueous phase can alter the phase chemistry and mixing dynamics in unpredictable ways. Particularly when making compositions at low final surfactant concentrations, the separation of the dilution step is a logical choice to reduce the uncertainty of the operation. However, there are situations in which having other ingredients present in substantial quantities in the aqueous phase during the surfactant dilution is actually preferred.
It has been surprisingly found that many of the common ingredients in cleaning compositions are actually not barriers to successful dilution of the concentrated lamellar surfactant, provided that care is taken to control of the flow ratio in the dilution operation. In fact, the viscosity-building aspect of these aqueous ingredients can improve the control of the flow ratio that is critical to avoiding mesophase production. The key breakthrough to implementing in the invention is the understanding of the influence of the aqueous phase comprising more than just water on the surfactant phase behavior, and therefore the range of flow ratios which leads to an acceptable cleaning composition or base for a cleaning composition exiting the mixing device.
The present invention eliminates the need for a separate dilution operation and allows for maximum flexibility in the relative compositions of various components in the cleaning composition. The skilled practitioner will recognize that the process described herein allows water that would normally be used strictly for dilution of the lamellar phase to be used for other purposes, such as polymer hydration or easier mixing of the other components into the cleaning composition. In some situations, the process may also allow for lower-temperature processing to achieve the final cleaning composition. Additionally, when a high concentration of surfactant is desired in the final cleaning composition, the present process improves on the current art by allowing for higher levels of other ingredients to be included in a cleaning composition and delayed addition of those ingredients, thereby enabling a wider range of possible formulas and operational logistics at the manufacturing facility. Later addition of ingredients into the process can be helpful for shear-sensitive ingredients and for improving operational logistics when making several products that differ only slightly from each other.